An X-ray tube is a controllable X-ray source, in which electrons detached from a cathode get accelerated in an electric field and hit an anode, where they lose their kinetic energy in various interaction processes with the atoms of the anode material. One result of these interaction processes is the generation of X-rays, the spectrum of which comprises both a continuous part (known as bremsstrahlung) and some prominent peaks. The energies at which the peaks occur depend on the anode material, because the peaks are associated with the relaxation of excited states in the atoms of the anode. Widely used anode materials are include (without being limited to) chromium, copper, molybdenum, rhodium, silver and tungsten. The spectral distribution and intensity of the bremsstrahlung part is proportional to both the acceleration voltage and the atomic ordinal number of the anode material: higher acceleration voltages and heavier anode materials increase the intensity of the continuous spectrum part at higher energies.
An X-ray tube is either of the bulk anode type or of the transmission anode type. A bulk anode is relatively thick and typically designed to direct the generated X-rays out of a separate window in a side surface of the X-ray tube, for which reason also the designation “side window type” is used for these kinds of X-ray tubes. A transmission anode is thin enough to let the generated X-rays pass through it. A transmission anode is typically a thin metal layer on an inner surface of an end window of the X-ray tube, giving rise to the alternative designation “end window type” X-ray tube.
The bremsstrahlung part and peak parts of the excitation spectrum are useful for different purposes for example in X-ray fluorescence analysis, in which the incident X-rays coming from an X-ray tube in turn excite the constituent particles of a target material. The fluorescence analysis involves detecting the fluorescent X-rays that come from the relaxation of excited states in said constituent particles, and using the detection results to make deductions about the presence of various elements in the target. The target may be very heterogeneous in constitution, like a soil sample from which the content of heavy metal pollutants should be measured. The characteristic peaks in the excitation radiation are useful for determining the matrix of ordinary soil constituents, while the high-energy bremsstrahlung part of the spectrum suitably excites the atoms of the heavy metals like lead, cadmium and others.
A problem with selecting the anode material occurs, because an anode material that gives good characteristic peaks does not necessarily give enough bremsstrahlung in the desired energy ranges. As an example we may consider rhodium as anode material. The so-called K lines of rhodium are easily applicable to determining the ratio between coherent scattering and Compton scattering, which enables using effective analytical tools for determining the matrix of a sample, such as soil. However, the amount of bremsstrahlung coming from a rhodium anode is relatively low in the frequency range that would be required to properly excite the atoms of cadmium, which is a typical pollutant to be measured from soil. The intensity of fluorescent radiation that can be obtained from a target material is proportional to the intensity of excitation radiation in the proper frequency range. Thus using a rhodium anode results in a relatively low intensity of fluorescent radiation from cadmium and other heavy metals, which weakens the analytical performance of the X-ray fluorescence analyzer in measuring soil pollution.